the BootLoader downgrader is a process that wipes BL 4.6 and puts on 3.9 - with a software method - however, this is a VERY risky process, zibri announces that if it errors out, YOU ARE SCREWED. you will not have a "brick" that can be restored. no. your iPhone will be destroyed.
I wouldn't click the downgrade BL button, just click jailbreak, activate, and unlock... that is all that you really need if you want to unlock your 4.6 phone.
until zibri makes the BL downgrade have a higher success rate, I won't be going anywhere near it.
EDIT : zibri has posted 2.2 which supposedly makes the downgrade BL more reliable.. I don't know why, but I still wouldn't touch that function. I mean really, why do you want 3.9? to unlock with anySIM? purpose? what was that? there isn't one - just stay at 4.6.